Liquid crystal display (LCD) technology is being developed as a possible successor to cathode ray (CRT) technology for many applications. LCD technology offers important advantages, such as higher reliability and reduced power, size and weight. However, in the current state of development, LCD image rendering capability falls short of that being achievable using CRTs. The present invention addresses a technical obstacle in LCD's which is vertical streaking that appears on display screens, especially when the video signal calls for a black screen.
The cause of the streaking problem can be better understood after a short discussion on the operation of a typical liquid crystal display. As is well known, an LCD is made up of a series liquid crystal cells aligned in rows and columns. Row and column lines run between the liquid crystal cells and carry voltage signals which turn on and off particular cells according to an incoming video signal. The amount a particular picture element turns on is controlled by the voltage level of the column line. For example, 0 volts on the column line may be a completely "off" (black) picture element and 20 V may be a completely "on" picture element. The voltage signals are provided to the column lines by the column driver. The column driver receives the raw video signal as well as various clock and sync pulses, and outputs voltage signals in synchrony with a row driver such that the picture elements are activated in a raster scan format as in a CRT. One element per column (but many columns per row) is activated at a time and the image is continually refreshed.
The driver mechanism in an LCD is typically comprised of a series of interconnected integrated circuits (IC's). Each IC is responsible for transmitting an image signal over a set number of columns. During operation of the display, voltage errors are introduced into the column lines from a variety of sources. Any electronic component within the driver has the potential to add even a minimal voltage to the signal sent out over the column lines. Because a different IC drives a different set of columns, the subtle differences among the IC's can result in different voltage levels being transmitted over the columns.
As an example, assume that there is a linear panel that has a peak gray (white) when driven by a 20 volt signal. With such a display, a just noticeable difference (JND) would appear if the line were driven approximately 0.12 volts lower. This can be derived from human vision models. According to these models, the worst case scenario occurs when displaying black because less intensity change is necessary for a JND. That intensity can correspond to a column line being driven at 0.006 volts higher. Performance requirements predicate that the voltage range be limited to .+-.3 millivolts over a range of 0-18 volts for graphics. Holding this tolerance can be difficult especially in light of the fact that the standard CMOS op amps used in the drivers typically exhibit .+-.150 millivolts offset. Drivers using an array of switches and precision voltage sources have become the method of choice, but this becomes clumsy as analog gray scale behavior (or a large number of gray levels) is approached. Therefore, the goal in the design of the electronics for a liquid crystal display is eliminating or significantly reducing the error voltage over the range of operation for the driver.
It is the object of the present invention to provide voltage offset compensation for a liquid crystal display so as to eliminate streaking over the display's full operating range.